The present invention relates to implant fixation systems, and particularly to systems for engaging an elongated member, such as a spinal rod, to a bone. The invention further pertains to a system utilizing a poly- or multi-axial bone fastener.
In many orthopaedic procedures, an implant is fixed to a bone to stabilize the bone. One example is for stabilization of a spinal segment. In one type of spinal fixation system, an elongated stabilization member, such as a spinal rod, is engaged to adjacent vertebrae by a bone engaging member. The bone engaging member is typically a hook configured to engage certain portions of the vertebral anatomy, or a bone screw adapted to be threaded into vertebral bone.
In orthopaedic stabilization systems, and most particularly in spinal systems, there is a need to accommodate various angular orientations between the elongated member and the spinal segment. This necessarily requires an ability to achieve variable angles between the bone engaging member and the elongated stabilization member. In the simplest case, the variable angle is in a single plane. In more complex cases, multi- or poly-axial angular orientations are required. One form of spinal stabilization system utilizes a bone screw having a generally spherically-shaped head. The screw head is seated within a mating cavity in a yoke. The yoke includes opposite arms forming a U-shaped slot for receipt of a spinal rod above the screw head.
Various mechanisms have been developed to lock the screw head and the rod within the yoke. In one such system, an insert is disposed between the bone screw head and the spinal rod. A set screw threaded into the arms of the yoke presses the rod into the insert, which presses the screw head into the yoke cavity. When fully tightened, the set screw effectively locks all of the components of the stabilization assembly together. The strength or tightness of this fixation requires that all of the components be properly oriented, otherwise no amount of tightening of the set screw will fully lock the assembly together. Since the components of the fixation system are typically assembled in situ, the surgeon's ability to visualize the instrumentation is limited. There is therefore a need for an implant fixation system that can provide assurances to the surgeon that the components are properly situated so that the components can be effectively locked together when the instrumentation is complete.